Method for manufacturing paramagnetic garnet-type transparent ceramics and method for suppressing insertion loss fluctuation within the optical effective diameter of paramagnetic garnet-type transparent ceramics

Abstract

To provide a method of producing paramagnetic garnet type transparent ceramic. i.e., a sintered body of a garnet-type composite oxide containing terbium and aluminum, mountable on a high-power laser system of 100 watts or more and usable as a magneto-optical device excellent in optical transparency.SOLUTION: The method of producing paramagnetic garnet type transparent ceramic by pressure-sintering a sintered body of a garnet type composite oxide containing terbium and aluminum, i.e., a columnar sintered body containing SiO2 in the amount of more than 0 mass% and 0.1 mass% or less as a sintering aid and applying oxidation annealing to the pressure-sintered body, comprises the steps of: converting the pressure-sintered body into a near-net shaped pressure-sintered body with a finishing diameter D1 after sintering within a range of +0.1 mm or more and 1.5 mm or less relative to a final diameter D2 of the paramagnetic garnet type transparent ceramic to be finally obtained; grinding the pressure-sintered body so that the diameter thereof becomes the desired final diameter; and subsequently applying oxidation annealing to the pressure-sintered body.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a paramagnetic garnet-type transparent ceramic containing terbium, which is suitable for forming magneto-optical devices such as optical isolators, and a method for producing the paramagnetic garnet-type transparent ceramic. Method for suppressing the fluctuation of insertion loss within the optical effective diameter of Regarding. [Background technology]

[0002] In recent years, as it has become possible to achieve higher output, the use of laser processing machines using fiber lasers has become increasingly common. However, when external light enters the laser light source built into the laser processing machine, the resonance state becomes unstable, causing the oscillation state to be disrupted. In particular, when the oscillated light is reflected by the optical system along the way and returns to the light source, the oscillation state is significantly disrupted. To prevent this, an optical isolator is usually installed before the light source.

[0003] An optical isolator consists of a Faraday rotator, a polarizer placed on the light input side of the Faraday rotator, and an analyzer placed on the light output side of the Faraday rotator. The Faraday rotator is used by applying a magnetic field parallel to the light's direction of propagation. The polarization of the light rotates in only one direction, whether it is traveling forward or backward through the Faraday rotator. Furthermore, the Faraday rotator's length is adjusted to rotate the polarization of the light exactly 45 degrees. If the polarization planes of the polarizer and analyzer are offset 45 degrees from the direction of the forward light's rotation, the forward light's polarization will be identical at the polarizer and analyzer positions, allowing it to pass through. On the other hand, the backward light's polarization will be rotated 45 degrees in the opposite direction to the polarizer's polarization, which is offset 45 degrees from the analyzer position. In this case, the polarization of the returning light at the polarizer position will be offset by 45 degrees - (-45 degrees) = 90 degrees from the polarizer's polarization, preventing it from passing through the polarizer. In this way, it functions as an optical isolator that transmits and emits forward-moving light and blocks backward-moving returning light.

[0004] The material used for the Faraday rotator that constitutes the optical isolator has traditionally been TGG crystal (Tb3Ga5O12 ) and TSAG crystal ((Tb (3-x) Sc x )Sc2Al3O 12 ) is known (Japanese Patent Laid-Open No. 2011-213552 (Patent Document 1), Japanese Patent Laid-Open No. 2002-293693 (Patent Document 2)). TGG crystals are currently widely used in standard fiber laser devices. On the other hand, the Verdet constant of TSAG crystals is said to be about 1.3 times that of TGG crystals, making them suitable for use in fiber laser devices. However, Sc is an extremely expensive raw material, and their adoption has not progressed due to manufacturing costs. Since then, development of TSAG crystals has continued, as in Japanese Patent No. 5611329 (Patent Document 3) and Japanese Patent No. 5935764 (Patent Document 4), but neither has been able to reduce the amount of Sc used, and they have not yet become widespread.

[0005] In addition to the above, TAG crystal (Tb3Al5O 12 ) is also known. However, because TAG crystals are decomposed melting crystals, there is a restriction that the perovskite phase is generated first at the solid-liquid interface, followed by the TAG phase. In other words, the garnet and perovskite phases of TAG crystals can only be grown in a state where they are always mixed together, and the growth of high-quality, large-sized TAG crystals has not yet been realized.

[0006] As a means of suppressing this mixed crystal formation, Japanese Patent Publication No. 3642063 (Patent Document 5) and Japanese Patent Publication No. 4107292 (Patent Document 6) propose a method of making the polycrystalline feed rod for FZ growth or the seed crystal porous, thereby preferentially precipitating the initial perovskite phase in the porous medium. However, in reality, as the melting position moves, the position where the perovskite phase is likely to precipitate also moves, so it is essentially impossible to completely suppress the precipitation of the perovskite phase simply by making the interface between the seed crystal and the polycrystalline feed rod porous.

[0007] Despite these constraints, Japanese Patent Laid-Open Publication No. 2008-7385 (Patent Document 7) proposes a material in which an oxide with a TAG composition is made into a ceramic, and yet is also translucent. Ceramics can be sintered at temperatures 100°C or lower than the melting point, which makes it possible to overcome the problem of decomposition and melting that is a problem with single crystal growth. Since TAG actually begins to decompose at temperatures above 1,840°C, if it can be sintered and densified to the maximum theoretical density below this temperature, it will be possible to obtain a transparent sintered body of single-phase TAG.

[0008] Patent Document 7 describes a method for producing ceramics having a garnet structure and made of terbium-aluminum oxide, which includes a step of mixing raw materials, a step of calcining, a step of pulverizing the calcined powder, a step of molding, and a step of firing. In the step of crushing the calcined powder, the average particle size of the calcined powder after pulverization is 0.2 to 1.6 μm, and in the step of molding, the density after molding is 3.26 g / cm. 3 It is said that if this is the case, TAG ceramics with high light transmittance can be produced.

[0009] However, in Patent Document 7, the light transmittance was extremely insufficient, with a maximum linear transmittance of only 35% at a thickness of 1.5 mm. Incidentally, when TAG is used as a Faraday element such as an optical isolator, for example, for a 1.06 μm band laser, the element length required to rotate the light 45 degrees requires approximately 15 mm, which is approximately 10 times the length described in Patent Document 7. For a material that transmits only 35% of light at a thickness of 1.5 mm, extending the element length by 10 times would result in a transmittance of less than 0.01%, or nearly zero, rendering the material completely nonfunctional. In other words, even if a ceramic manufacturing method could suppress the generation of heterophases, no practical TAG had existed until now.

[0010] Patent Document 6 shows that substituting part of the Tb in a TAG crystal with Ce increases the Verdet constant compared to TAG. If the Verdet constant is increased, the element length required to rotate incident light by 45 degrees can be shortened, reducing the total absorption amount. However, if the linear transmittance at a thickness of 1.5 mm is 35%, even if the element length is halved, the 45-degree rotation thickness transmittance will be less than 1%, making it far from practical use.

[0011] In addition, in the above situation, recently, the composition (Tb x Y 1-x )3AlO 12 It has been disclosed that dense ceramic sintered bodies with a (0.5≦x≦1.0) extinction ratio have a higher extinction ratio than existing TGG crystals (improving from 35dB of existing TGG crystals to 39.5dB or more) and can also reduce insertion loss (improving from 0.05dB to 0.01-0.05dB) (Yan Lin Aung, Akio Ilkesue, Development of optical grade (Tb x Y 1-x )3AlO 12 Ceramics as Faraday rotator material, J.Am.Ceram.Soc., (2017), 100(9), 4081-4087 (Non-Patent Document 1). The material disclosed in Non-Patent Document 1 is, first of all, a ceramic, so it does not suffer from the precipitation of perovskite heterophases that was a problem with TGG crystals, and furthermore, by substituting some of the Tb ions with Y ions, it is possible to further reduce loss, making it a material that can be used to obtain extremely high-quality garnet-type Faraday rotators. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-213552 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-293693 [Patent Document 3] Patent No. 5611329 [Patent Document 4] Patent No. 5935764 [Patent Document 5] Patent No. 3642063 [Patent Document 6] Patent No. 4107292 [Patent Document 7] Japanese Patent Application Laid-Open No. 2008-7385

[0013] [Non-Patent Document 1] Yan Lin Aung, Akio Ilkesue, Davelopment of optical grade (TbxY1-x)3Al5O12 Ceramics as Faraday rotator material, J.Am.Ceram.Soc., (2017),100(9),4081-4087 Summary of the Invention [Problem to be solved by the invention]

[0014] However, when the inventors actually performed follow-up tests on the material described in Non-Patent Document 1, they confirmed that a high-quality ceramic sintered body with a lower insertion loss than TGG crystals could indeed be obtained, but they also confirmed that the reproducibility was poor, and that even though the insertion loss was lower than that of TGG crystals, the focal position changed by 0.2 m or more when a high-power laser of 100 W or more was irradiated (thermal lens phenomenon).

[0015] The present invention has been made in view of the above circumstances, and provides a method for producing a paramagnetic garnet-type transparent ceramic, which is a sintered body of a garnet-type composite oxide containing terbium and aluminum, and which can be used as a magneto-optical device having truly excellent optical transparency and can be installed in a high-power laser system of 100 W or more, and a method for producing the paramagnetic garnet-type transparent ceramic. Method for suppressing the fluctuation of insertion loss within the optical effective diameter of The purpose is to provide. [Means for solving the problem]

[0016] In order to achieve the above object, the present invention provides the following method for producing a paramagnetic garnet-type transparent ceramic and a pressure-sintered body for producing the paramagnetic garnet-type transparent ceramic. 1. Garnet-type containing at least terbium and aluminum (A 3 B 5 O 12 ) composite oxides (However, the A site of the composition formula contains 60 mol % or more and 100 mol % or less of terbium, and the B site contains 80 mol % or more and 100 mol % or less of aluminum.) A method for producing a paramagnetic garnet-type transparent ceramic, comprising pressure-sintering a cylindrical sintered body containing more than 0 mass % and 0.1 mass % or less of SiO2 as a sintering aid, and further subjecting the pressure-sintered body to oxidation annealing, a pressure-sintered body having a near-net-shape diameter that is +0.1 mm to +1.5 mm relative to the final diameter of the paramagnetic garnet-type transparent ceramics to be obtained, the pressure-sintered body being ground to the final diameter, and then the oxidation annealing treatment is carried out. 2. 2. A method for producing a paramagnetic garnet-type transparent ceramic according to 1, wherein the garnet-type composite oxide further contains yttrium. 3. 3. The method for producing a paramagnetic garnet-type transparent ceramic according to 1 or 2, wherein the average particle size of the resulting paramagnetic garnet-type transparent ceramic is 10 μm or more. 4. 4. The method for producing a paramagnetic garnet-type transparent ceramic according to any one of 1 to 3, wherein the length of the near-net-shape pressure-sintered compact in the optical axis direction after pressure sintering is +0.5 mm or more relative to the final length in the optical axis direction of the paramagnetic garnet-type transparent ceramic to be finally obtained. 5. 5. The method for producing a paramagnetic garnet-type transparent ceramic according to any one of 1 to 4, wherein the final diameter of the paramagnetic garnet-type transparent ceramic obtained as a result is 3.6 to 8 mm. 6. Garnet-type containing at least terbium and aluminum (A 3 B 5 O 12 ) composite oxides (However, the A site of the composition formula contains 60 mol % or more and 100 mol % or less of terbium, and the B site contains 80 mol % or more and 100 mol % or less of aluminum.) a cylindrical sintered body containing more than 0% by mass and not more than 0.1% by mass of SiO2 as a sintering aid, the pressure sintered body being a near-net-shape type whose finished diameter after pressure sintering is +0.1 mm to 1.5 mm relative to the final diameter of the paramagnetic garnet-type transparent ceramic to be obtained, the outer periphery of which is ground to the final diameter, and then oxidation annealing is performed. 7. 7. A method for suppressing the fluctuation of insertion loss within the optically effective diameter of a paramagnetic garnet-type transparent ceramic according to 6, wherein the garnet-type composite oxide further contains yttrium. 8. 8. A method for suppressing the fluctuation of insertion loss within the optical effective diameter of a paramagnetic garnet-type transparent ceramic according to 6 or 7, wherein the length of the pressure-sintered compact in the optical axis direction is +0.5 mm or more relative to the final length in the optical axis direction of the resulting paramagnetic garnet-type transparent ceramic. 9. 9. The method for suppressing the fluctuation of insertion loss within the optically effective diameter of a paramagnetic garnet-type transparent ceramic according to any one of 6 to 8, wherein the final diameter of the paramagnetic garnet-type transparent ceramic obtained as a result is 3.6 to 8 mm. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a truly practical paramagnetic garnet-type transparent ceramic, which is a garnet-type composite oxide containing terbium and aluminum, has an extremely small loss coefficient, low scattering, and high quality, and can be used as a magneto-optical material with truly excellent optical transparency, which can be installed in high-power laser systems, particularly those of 100 W or more, and which can be easily scaled up because it is a ceramic sintered body. [Brief explanation of the drawings]

[0018] [Figure 1] 1A and 1B are schematic diagrams showing examples of a pressure-sintered compact in a pressure-sintered state in which the outer periphery is ground to obtain the final diameter. FIG. 1A is a near-net-shape pressure-sintered compact of the present invention, and FIG. 1B is a pressure-sintered compact with a large outer periphery grinding allowance. [Figure 2] FIG. 1 is a cross-sectional view showing an example of the configuration of an optical isolator using the paramagnetic garnet-type transparent ceramic obtained in the present invention as a Faraday rotator. DETAILED DESCRIPTION OF THE INVENTION

[0019] The method for producing a paramagnetic garnet-type transparent ceramic according to the present invention and the structure of a pressure-sintered body for producing the paramagnetic garnet-type transparent ceramic will be described below.

[0020] [Method for manufacturing paramagnetic garnet-type transparent ceramics] The method for producing a paramagnetic garnet-type transparent ceramic according to the present invention comprises pressure-sintering a cylindrical sintered body of a garnet-type composite oxide containing at least terbium and aluminum, the cylindrical sintered body containing more than 0% by mass and not more than 0.1% by mass of SiO2 as a sintering aid, and then subjecting the pressure-sintered body to an oxidation annealing treatment, characterized in that the pressure-sintered body is a near-net-shape pressure-sintered body whose diameter after pressure sintering is +0.1 mm to 1.5 mm relative to the final diameter of the paramagnetic garnet-type transparent ceramic to be obtained, and the pressure-sintered body is ground to the final diameter, followed by the oxidation annealing treatment.

[0021] <Paramagnetic garnet-type transparent ceramics> First, the paramagnetic garnet-type transparent ceramics produced in the present invention will be described. The paramagnetic garnet-type transparent ceramics produced by the present invention are sintered bodies of garnet-type composite oxides containing at least terbium (Tb) and aluminum (Al), and further contain more than 0 mass % and 0.1 mass % or less of SiO2 as a sintering aid. The garnet-type composite oxides preferably further contain yttrium.

[0022] In general, terbium tends to exist stably in a trivalent state in oxides with a garnet structure, which is preferable because it allows the production of highly transparent paramagnetic garnet-type ceramics with low absorption. Furthermore, incorporating aluminum into the garnet structure is preferable because it reduces the lattice constant and increases the Verdet constant per unit length. Furthermore, adding more than 0% by mass and up to 0.1% by mass of SiO2 during sintering promotes densification and produces highly transparent sintered bodies, which is also preferable. Furthermore, SiO2 is expected to have the effect of suppressing the formation of heterophases such as perovskite, so its active addition is preferable.

[0023] Adding more than 0.1 mass% of SiO2 is not desirable because the maximum change in focal position due to the thermal lens phenomenon exceeds 0.25 m when a 100 W laser beam with a wavelength of 1,064 nm is irradiated onto a paramagnetic garnet-type transparent ceramic with a length (optical path length) of 15 to 25 mm (this required optical path length varies depending on the outer diameter of the optical element used and the size of the magnet covering it).

[0024] In the present invention, in addition to the above elements, yttrium (Y), lutetium (Lu), scandium (Sc), gallium (Ga), cerium (Ce), magnesium (Mg), calcium (Ca), etc. can also be suitably added.

[0025] Substituting some of the terbium sites with yttrium or lutetium is preferable because it stabilizes the garnet structure and reduces defects. However, substituting too much of these elements for terbium is undesirable because it unnecessarily reduces the Verdet constant of the resulting sintered body. Yttrium and lutetium are typically substituted in the range of 0 mol% to 40 mol%, and more preferably in the range of 5 mol% to 40 mol%, assuming terbium is 100 mol%.

[0026] Scandium is a material with an intermediate ionic radius that can dissolve in both the terbium site and some of the aluminum site in oxides having a garnet structure. For example, if the resulting composition deviates from the stoichiometric ratio due to variations in the weighing of various starting materials, scandium can adjust its distribution ratio to the rare earth sites consisting of terbium and yttrium and the aluminum site to form a solid solution so that the composition matches the stoichiometric ratio and thereby minimize the energy required to form crystallites. In other words, scandium is an element that can stably obtain a single garnet phase, so it can be suitably added.

[0027] However, adding too much scandium increases the substitution rate of terbium, resulting in an unnecessary decrease in the terbium solid solution concentration. This is undesirable because it reduces the Verdet constant. Furthermore, because scandium is expensive as a raw material, unnecessary excessive doping with scandium is undesirable from the viewpoint of production costs. Typically, scandium is preferably substituted in the range of 0 mol% to 20 mol% and more preferably 0 mol% to 5 mol% when aluminum is taken as 100 mol%.

[0028] Cerium, when present in terbium oxide as a solid solution, can extend the absorption range to wavelengths longer than 488 nm and increase the spin-orbit coupling splitting by breaking the symmetry degeneracy. This increases the Verdet constant when laser light with a wavelength of 1,064 nm is incident. Therefore, it is a preferred element to add in the present invention. However, if it is substituted in too large a quantity, the influence of absorption at a wavelength of 1,064 nm becomes unnegligible, which is undesirable. Typically, cerium is substituted in a range of 0 mol% to 1 mol% and more preferably 0 mol% to 0.5 mol% of terbium, assuming 100 mol% terbium.

[0029] Magnesium and calcium are both divalent ions, and can compensate for the charge balance shift within the garnet structure that occurs when SiO2, which is tetravalent, is added, so they can be suitably added. The amount of magnesium and calcium added is preferably adjusted to match the amount of SiO2 added.

[0030] The paramagnetic garnet-type transparent ceramics produced by the present invention contain the above-mentioned element group as a main component. Here, "containing as a main component" means that the composite oxide composed of the above-mentioned element group is contained in an amount of 90% by mass or more. In this case, the content is preferably 99% by mass or more, more preferably 99.9% by mass or more, even more preferably 99.99% by mass or more, and particularly preferably 99.999% by mass or more.

[0031] The paramagnetic garnet-type transparent ceramics produced by the present invention may contain other elements in addition to the above-mentioned main components. Typical examples of other elements include sodium (Na), phosphorus (P), tungsten (W), molybdenum (Mo), etc. The content of other elements is preferably 10 parts by mass or less, more preferably 0.1 parts by mass or less, and particularly preferably 0.001 parts by mass or less (substantially zero), when the total amount of terbium is taken as 100 parts by mass.

[0032] In the present invention, the above-mentioned paramagnetic garnet-type transparent ceramics are produced as follows.

[0033] [Raw materials] The raw materials used in the present invention include metal powders containing at least terbium and aluminum, and optionally yttrium, lutetium, scandium, gallium, cerium, magnesium, calcium, etc., or aqueous solutions of nitric acid, sulfuric acid, uric acid, etc., or oxide powders of the above elements. The purity of the raw materials is preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more.

[0034] In addition, silicon oxide powder or TEOS (tetraethyl orthosilicate) can be suitably used as the SiO2 raw material used as the sintering aid in the present invention. The purity of the raw material is preferably 99.9 mass% or more, and particularly preferably 99.99 mass% or more.

[0035] The above elements are weighed out in predetermined amounts (composition ratios of the desired paramagnetic garnet-type transparent ceramics), mixed, and pulverized to obtain a starting raw material powder. In this case, the mixed raw materials may be fired once and then pulverized again to use as the starting raw material powder. The firing temperature is not particularly limited as long as it is 1,200°C or lower.

[0036] The shape of the starting powder is not particularly limited, but powders that are as spherical as possible are preferred because they offer superior moldability, fluidity, and sinterability. Secondary agglomerated powders can also be used. However, these secondary agglomerated particles must be able to be crushed to a certain extent in the subsequent uniaxial press process. Therefore, it is preferable to appropriately adjust the ratio of binder and dispersant among the organic additives described below to find a range in advance where the crushing strength of the secondary agglomerated particles is equal to or less than the uniaxial press pressure, and use raw materials that satisfy these conditions as starting materials.

[0037] The preparation process of these raw material powders is not particularly limited. Raw material powders prepared by any synthesis method, such as coprecipitation, pulverization, spray pyrolysis, sol-gel, or alkoxide hydrolysis, can be suitably used. Furthermore, the obtained raw material powders may be processed appropriately using a wet ball mill, bead mill, jet mill, dry jet mill, hammer mill, or the like. However, to avoid contamination with metal impurities, especially Fe, it is particularly preferable to use a wet ball mill, bead mill, or wet jet mill for the dispersion treatment.

[0038] Various organic additives may be added to the raw material powder used in the present invention for the purpose of improving quality stability and yield in the subsequent ceramic manufacturing process. In the present invention, it is preferable to actively utilize these additives. That is, it is preferable to actively utilize dispersants, binders, lubricants, plasticizers, etc. However, adding excessive binder generally results in excessive hardening of the resulting raw material powder. Therefore, it is necessary to confirm the appropriate binder addition ratio in advance. It is preferable to select high-purity organic additives that do not contain unnecessary metal ions.

[0039] [Manufacturing process]

[0040] In the present invention, the above-mentioned raw material powder is press-molded, debound, and then sintered to produce a sintered body with a relative density of at least 94%. This is followed by a hot isostatic pressing (HIP) process. If the HIP process is directly performed, the paramagnetic garnet-type transparent ceramics will be reduced, resulting in some oxygen deficiency. Therefore, it is preferable to perform a slight-oxidizing HIP process, or an annealing process in an oxidizing atmosphere (oxidizing annealing process) after the HIP process to recover the oxygen deficiency. This allows for the production of a transparent garnet-type ceramics with no defect absorption.

[0041] (molding) In the present invention, the raw material powder is press-molded into a cylindrical shape of predetermined dimensions. The "predetermined dimensions" here refer to a shape obtained by press-molding using a mold jig having an inner diameter designed by back-calculating so that the outer diameter after densification, i.e., the pressure-sintered finished diameter, is the final finished outer diameter, i.e., the final diameter of the paramagnetic garnet-type transparent ceramic to be obtained, to obtain a near-net-shape pressure-sintered compact (HIP compact) having an inner diameter that is 0.1 mm to 1.5 mm, preferably 0.1 mm to 1.0 mm, and more preferably 0.1 mm to 0.5 mm. Empirically, it is preferable to use a press mold jig with an inner diameter that is approximately 25 to 35% larger than the final finished outer diameter (final diameter), but the specific dimensions are preferably determined through verification experiments of several batches.

[0042] In the present invention, a conventional press molding process can be suitably used. That is, a very common uniaxial pressing process in which a material is filled into a mold and pressure is applied from a specific direction can be suitably used. The inner diameter of the uniaxial pressing jig is preferably strictly specified as described above. Furthermore, the length of the uniaxial pressing jig must be long enough to remove a pressed body that is 25 to 35% longer than the final length of the paramagnetic garnet-type transparent ceramic obtained after uniaxial pressing, plus at least 0.5 mm. Typically, it is preferable to ensure a length at least twice the length of the desired pressed body. In the present invention, a cold isostatic pressing (CIP) process or a warm isostatic pressing (WIP) process may be added after the uniaxial pressing process. The applied pressure can be adjusted appropriately while checking the relative density of the resulting green body and is not particularly limited. However, for example, controlling the pressure within a pressure range of approximately 300 MPa or less, which is compatible with commercially available CIP and WIP equipment, can reduce production costs.

[0043] Alternatively, instead of press molding, green bodies can be produced by slip casting. Other methods, such as pressure casting, centrifugal casting, and extrusion molding, can also be employed by optimizing the shape and size of the starting oxide powder and the combination of various organic additives. However, in the present invention, the dimensions of the green body are controlled so that the finished diameter of the near-net-shape pressure-sintered compact (HIP compact) after pressure sintering falls within a certain range that is slightly larger than the final outer diameter (final diameter), as described above. Therefore, even when using slip casting, the internal dimensions of the molding jig must be precisely controlled by back-calculation.

[0044] (Degreasing) In the present invention, a conventional debinding process can be suitably used. That is, a temperature-raising debinding process using a heating furnace can be performed. The type of atmospheric gas used is not particularly limited, and air, oxygen, hydrogen, etc. can be suitably used. The debinding temperature is also not particularly limited, but if a raw material containing an organic additive is used, it is preferable to raise the temperature to a temperature at which the organic component can be decomposed and eliminated.

[0045] (sintering) In the present invention, a general sintering process can be suitably used. That is, a heat sintering process such as a resistance heating method or an induction heating method can be suitably used. The atmosphere used is not particularly limited, and sintering can be carried out in various atmospheres such as an inert gas, oxygen gas, hydrogen gas, or helium gas, or under reduced pressure (vacuum). However, since it is preferable to prevent the occurrence of oxygen deficiency in the final product, an oxygen gas or reduced-pressure oxygen gas atmosphere is more preferred.

[0046] The sintering temperature in the sintering step of the present invention is preferably 1,390 to 1,750° C., particularly preferably 1,390 to 1,730° C. A sintering temperature in this range is preferable because it promotes densification while suppressing heterogeneous phase precipitation in the sintered body.

[0047] In the sintering process of the present invention, a sintering holding time of several hours is sufficient, but the relative density of the sintered body must be densified to at least 94% or more and must not exceed 99%. Proceeding sintering until the relative density of the sintered body exceeds 99% is not desirable because it would result in a rapid increase in the amount of bubbles remaining inside the final sintered body. In other words, the relative density of the sintered body is preferably 94% to 99%, and more preferably 94.5% to 99.0%. Furthermore, in the present invention, the average loss factor of the obtained sintered body is 0.0017 cm -1 In order to keep this value below 10 μm, it is preferable to adjust the sintering conditions so that the final average grain size (that is, the average grain size of the pressure-sintered compact) is 10 μm or more.

[0048] (Hot Isostatic Pressing (HIP)) In the present invention, after the sintering step, a hot isostatic pressing (HIP) treatment is further carried out.

[0049] The type of pressurized gas medium that can be used here is preferably an inert gas such as argon or nitrogen, or Ar-O2. The pressure applied by the pressurized gas medium is preferably 50 to 300 MPa, more preferably 100 to 300 MPa. If the applied pressure is less than 50 MPa, the transparency improvement effect may not be obtained, and if the applied pressure exceeds 300 MPa, further transparency improvement cannot be obtained even if the pressure is increased, and the load on the equipment may be excessive, which may damage the equipment. For convenience, the applied pressure is preferably 196 MPa or less, which can be processed with commercially available HIP equipment.

[0050] The treatment temperature (also referred to as the predetermined holding temperature or HIP treatment temperature) is set in the range of 1,100 to 1,780°C, preferably 1,200 to 1,730°C. A heat treatment temperature above 1,780°C is not preferred because it increases the risk of oxygen vacancies. A heat treatment temperature below 1,100°C does not achieve much of an effect of improving the transparency of the sintered body. There are no particular restrictions on the holding time at the heat treatment temperature, but holding for too long a time is not preferred because it increases the risk of oxygen vacancies. Typically, it is preferably set in the range of 1 to 3 hours.

[0051] The heater material, insulator, and processing vessel used in the HIP process are not particularly limited, but graphite, molybdenum, tungsten, and platinum are suitable, and yttrium oxide and gadolinium oxide are also suitable for the processing vessel. Platinum can be used for the heater material, insulator, and processing vessel, and Ar-O can be used as the pressurized gas medium, which is particularly preferred when the processing temperature is 1,500°C or lower, because this prevents oxygen deficiency during the HIP process. When the processing temperature exceeds 1,500°C, graphite is preferred for the heater material and insulator. In this case, however, it is preferable to select graphite, molybdenum, or tungsten for the processing vessel, and further select yttrium oxide or gadolinium oxide as a double container inside, and fill the container with an oxygen-releasing material, because this minimizes the amount of oxygen deficiency during the HIP process.

[0052] In the present invention, the grain size of the sintered body sintered in the previous step is further grown in the HIP treatment step, thereby making it possible to make the final sintered grain size 10 μm or more. Note that the average sintered grain size (average grain size) of the pressure-sintered body obtained by this HIP treatment is not changed by the oxidation annealing treatment described below, and therefore becomes the average sintered grain size (average grain size) of the finally obtained paramagnetic garnet-type transparent ceramics.

[0053] The average sintered grain size of the pressure-sintered body or the paramagnetic garnet-type transparent ceramic is determined by measuring the grain size of the sintered grains of the target paramagnetic garnet-type transparent ceramic with a metallurgical microscope, and is determined in detail as follows. Using the transmission mode of a metallurgical microscope and a 50x objective, transmission open Nicol images of a paramagnetic garnet-type transparent ceramic sample with both end faces polished are taken. Specifically, the entire optically effective area of the target paramagnetic garnet-type transparent ceramic is photographed, taking into account the effective image size and effective focal depth of the objective, and the photographed images are then analyzed. First, a diagonal line is drawn on each photographed image, and the total number of sintered particles crossed by the diagonal line is counted. The average diameter of the sintered particles in that image is then defined as the sum of the average diameters of the photographed images read in the analysis process, and then divided by the number of photographs (the same applies below).

[0054] (Pressure sintered compact for manufacturing paramagnetic garnet-type transparent ceramics) In the present invention, a pressure sintered body is obtained by HIP treatment. As described above, this pressure-sintered compact is a cylindrical sintered compact made of a garnet-type composite oxide containing at least terbium and aluminum, and containing more than 0 mass % and not more than 0.1 mass % of SiO2 as a sintering aid, and is a near-net-shape pressure-sintered compact for producing paramagnetic garnet-type transparent ceramics, the finished diameter of which after pressure sintering is +0.1 mm or more and not more than 1.5 mm relative to the final diameter of the paramagnetic garnet-type transparent ceramics to be obtained.

[0055] The pressure-sintered compact obtained by the present invention is a near-net-shape pressure-sintered compact in which the diameter after pressure sintering is larger than the final diameter (final outer diameter) of the paramagnetic garnet-type transparent ceramic to be finally obtained, but is finished to a diameter as close as possible to the final diameter. Furthermore, it is preferable that the length after pressure sintering in the optical axis direction of the near-net-shape pressure-sintered compact is longer than the final length in the optical axis direction of the paramagnetic garnet-type transparent ceramic to be finally obtained (final finished length), but is finished to a length as close as possible to the final finished length, i.e., a pressure-sintered compact processed and adjusted to a near-net shape.

[0056] Figure 1 shows an example of a pressure-sintered compact in its final sintered state after grinding its outer periphery to obtain the final diameter. Figure 1(a) shows a near-net-shape pressure-sintered compact of the present invention, while Figure 1(b) shows a pressure-sintered compact with a large outer periphery grinding allowance, shown for comparison. Sintering at dimensions as close to the final size as possible, as in the near-net-shape pressure-sintered compact of the present invention shown in Figure 1(a), is preferable because it minimizes the diffusion distance required for bubble evacuation during sintering and pressure sintering, reducing the likelihood of residual bubbles. In particular, with regard to the diameter (outer diameter), it is preferable to make the sintered finished diameter D1 as close as possible to the final diameter (final finished outer diameter) D2, as this reduces residual bubbles in the center of the pressure-sintered compact. The center of the sintered finished diameter D1 and the final diameter (final finished outer diameter) D2 are the same.

[0057] The finished diameter D1 of the near-net-shape pressure-sintered compact 10 of the present invention is +0.1 mm to +1.5 mm, preferably +0.1 mm to +1.0 mm, and more preferably +0.1 mm to +0.5 mm, of the final diameter D2 of the paramagnetic garnet-type transparent ceramic 11 obtained. That is, the cutting allowance t, which is half the difference between the finished diameter D1 and the final diameter D2, is 0.05 mm to 0.75 mm, preferably 0.05 mm to 0.5 mm, and more preferably 0.05 mm to 0.25 mm. When the difference between the finished diameter D1 and the final diameter D2 of the pressure-sintered compact, or the cutting allowance t, is within this range, residual bubbles can be suppressed throughout the entire optical effective diameter, including the center of the pressure-sintered compact, where residual bubbles are most likely to remain.

[0058] If the diameter after pressure sintering is less than plus (+)0.1 mm relative to the final diameter D2 of the resulting paramagnetic garnet-type transparent ceramic, the warping characteristic of elongated pressure sintered compacts with a high aspect ratio will prevent the required diameter (optical effective diameter) for a magneto-optical device, particularly a Faraday rotator, from being achieved over the entire length from one end to the other in the optical axis direction of the pressure sintered compact. On the other hand, as shown in Figure 1(b), if the diameter D1' after pressure sintering is more than plus (+)1.5 mm relative to the final diameter D2 of the resulting paramagnetic garnet-type transparent ceramic (i.e., if the cutting allowance t' is more than 0.75 mm), the amount of residual bubbles inside the pressure sintered compact 90, especially at the center of the optical effective diameter of the paramagnetic garnet-type transparent ceramic 91 after cutting, will increase significantly, making the pressure sintered compact unsuitable.

[0059] Furthermore, the final length in the optical axis direction (final finished length) of the paramagnetic garnet-type transparent ceramic to be finally obtained varies depending on the outer diameter of the target magneto-optical device (optical element) and the size of the magnet covering it. However, in any case, it is preferable that the final pressure-sintered length in the optical axis direction of the near-net-shape pressure-sintered compact is at least +0.5 mm longer than the final length in the optical axis direction of the paramagnetic garnet-type transparent ceramic to be finally obtained. There is no particular upper limit. This is because the bubble discharge during the sintering and pressure-sintering is achieved mainly by radial diffusion and therefore is hardly dependent on the longitudinal length. However, a compact with a high aspect ratio, in which the length in the optical axis direction (longitudinal direction) is increased while maintaining a specified diameter, may crack or warp during molding. Therefore, it is preferable to limit the final pressure-sintered length to the required length plus 5 mm or less to avoid any additional problems.

[0060] The condition that the finished diameter of the pressure-sintered paramagnetic garnet-type transparent ceramic must be at least +0.1 mm greater than the final diameter of the resulting paramagnetic garnet-type transparent ceramic (i.e., the condition that the cutting allowance t must be at least 0.05 mm) does not apply to paramagnetic garnet-type transparent ceramics (optical elements) whose final diameter (final finished outer diameter) is 3 mm or less. There are two reasons for this. First, the paramagnetic garnet-type transparent ceramics (optical elements) defined in this invention, which are made of a sintered body of a garnet-type composite oxide containing at least terbium and aluminum, are intended for use as magneto-optical materials with truly excellent optical transparency that can be installed in high-power laser systems of 100 W or more, and magneto-optical materials for such high-power applications usually have a diameter of 3.5 mm or more. The second reason is that when attempting to form thin paramagnetic garnet-type transparent ceramics (optical elements) with a diameter of 3 mm or less into a near-net shape, the relative shear stress between the inner surface of the long, thin uniaxial press jig and the powder that is filled vertically to be formed increases rapidly, preventing proper packing and causing cracking.

[0061] Optical elements with a diameter of 3 mm or less are intended for low-power applications, but when producing such thin paramagnetic garnet-type transparent ceramics (optical elements), it is advisable to produce a large-diameter molded body and then divide it into many thin optical elements by post-processing. In this case, the intragranular bubbles in the pressure-sintered body will increase considerably compared to those of the present invention, but they can still often be used for low-power applications.

[0062] The final diameter of the paramagnetic garnet-type transparent ceramics finally obtained in the present invention is 3.6 mm or more, preferably 4 mm or more, and more preferably 5 mm or more. When the final diameter of the paramagnetic garnet-type transparent ceramics finally obtained is 5 mm or more, it is particularly preferable because it greatly exceeds 100 W and can be mounted on a high-power laser system of 150 W class. There is no particular upper limit to the final diameter of the paramagnetic garnet-type transparent ceramics finally obtained, but it is, for example, 8.0 mm.

[0063] The finished diameter D1 of the near-net-shape pressure-sintered compact 10 of the present invention is between plus (+) 0.1 mm and plus (+) 1.5 mm relative to the final diameter D2 of the paramagnetic garnet-type transparent ceramic 11. For example, if the final diameter D2 is 3.6 mm, the finished diameter D1 of the pressure-sintered compact will be between 3.7 mm and 5.1 mm; if the final diameter D2 is 4 mm, the finished diameter D1 of the pressure-sintered compact will be between 4.1 mm and 5.5 mm; and if the final diameter D2 is 5 mm, the finished diameter D1 of the pressure-sintered compact will be between 5.1 mm and 6.5 mm. Note that the finished diameter D1 of the pressure-sintered compact 10 after HIP (densification treatment) is the finished diameter (outer diameter) of the pressure-sintered compact 10; the outer diameter of the uniaxially pressed compact is about 35% larger than this, and the outer diameter of the compact before sintering (CIP compact) is about 20% larger than this.

[0064] (periphery grinding) Before subjecting the pressure-sintered compact obtained as described above to the oxidation annealing treatment described below, the outer periphery of the pressure-sintered compact is ground to a finishing process so that the outer diameter falls within the tolerance range of the outer diameter required for the magneto-optical device intended for final use. That is, as described above, for a near-net-shape pressure-sintered compact, whose finished diameter after pressure sintering is as close as possible to the final diameter (final finished outer diameter) of the paramagnetic garnet-type transparent ceramic obtained by the HIP treatment, the outer periphery of the pressure-sintered compact is preferably further ground to the desired final diameter (final finished outer diameter). In this case, there is no practical problem if the desired final outer diameter is achieved within the processing accuracy of a typical processing machine during the outer periphery grinding process, but it is preferable to finish the final diameter (final finished outer diameter) to within ±15 μm in actual value and ±30 μm in tolerance.

[0065] In the present invention, grinding can be suitably performed using a conventional centerless grinding machine, cylindrical grinding machine, etc. In particular, when attempting to produce truly transparent and practical paramagnetic garnet-type transparent ceramics that can be mounted on a high-power laser system of 100 W or more, the length in the optical axis direction must be 17 mm or more, and the sintered body will have a fairly long and narrow shape, so a centerless grinding machine is more suitable.

[0066] (oxidation annealing) In the present invention, the grinding process on the outer periphery of the pressure-sintered compact causes damage to the processed surface to a certain depth (a damaged layer is formed). Oxidation annealing is essential to remove (recover) this damage. Furthermore, the pressure-sintered compact often has oxygen deficiency due to the sintering process (including HIP process) in a reducing atmosphere. An oxidation annealing process is also required to recover from this. Therefore, in order to complete these oxidation annealing processes in one step, they are performed as a post-process after the grinding process on the outer periphery of the pressure-sintered compact.

[0067] That is, in the present invention, after grinding the outer periphery of the pressure-sintered compact, it is preferable to perform oxidation annealing in an air or oxygen atmosphere at a temperature equal to or lower than the HIP treatment temperature, typically 1,000 to 1,600° C., and preferably 1,100 to 1,500° C. This treatment makes it possible to simultaneously recover oxygen vacancies and recover the outer periphery damaged layer in the pressure-sintered compact after periphery grinding.

[0068] The holding time in the oxidation annealing step is not particularly limited, but it is preferable to select a time that is long enough to recover oxygen vacancies and that does not consume electricity due to unnecessary long-term treatment. Since the time required to recover oxygen vacancies is generally longer than the time required to recover the peripheral damaged layer, by ensuring a time that is long enough to recover the oxygen vacancies, the peripheral damaged layer will also recover at the same time. By performing the oxidation annealing treatment, it is possible to obtain truly transparent paramagnetic garnet-type transparent ceramics that are completely colorless and transparent and have no oxygen vacancies or peripheral damage defects.

[0069] Note that if the oxidation annealing step is performed at a high temperature outside the above range for a long period of time, the size and quantity of remaining bubbles inside the pressure-sintered compact may increase, making it difficult to ensure an extinction ratio of 40 dB or more across the entire optical effective diameter at a wavelength of 1,064 nm. However, in such a case, it is possible to reduce the number of remaining bubbles inside the pressure-sintered compact again by subjecting the pressure-sintered compact to HIP treatment again and then subjecting it to oxidation annealing treatment again.

[0070] In the present invention, a ground, pressure-sintered compact with an average sintered grain size (average grain size) of 10 μm or more is thoroughly annealed in air or an oxygen atmosphere to completely eliminate color center absorption due to oxygen vacancies, resulting in a paramagnetic garnet-type transparent ceramic with an extinction ratio of 40 dB or more across the entire optically effective surface at a wavelength of 1,064 nm and an optical path length of 15 mm to 25 mm. In this case, if the pressure-sintered compact has an average sintered grain size (average grain size) of 10 μm or more, oxidation annealing can easily ensure an extinction ratio of 40 dB or more across the entire optically effective surface. Conversely, if the pressure-sintered compact has an average sintered grain size (average grain size) of less than 10 μm, particularly less than 5 μm, oxidation annealing may result in some regions where the extinction ratio does not improve to 40 dB or more.

[0071] (optical polishing) It is preferable to optically polish both end faces of the pressure-sintered compact that has been subjected to the oxidation annealing treatment and the series of manufacturing steps described above. The optical surface precision at this time is preferably λ / 2 or less, and particularly preferably λ / 8 or less, when the measurement wavelength λ is 633 nm. Furthermore, it is preferable to always polish the surface to prevent surface scratches from remaining on the optically polished surface.

[0072] (anti-reflective coating) Next, it is preferable to further form an anti-reflection coating (AR coating) on the optically polished end facets. This treatment is preferable because it can minimize the optical loss of the resulting paramagnetic garnet-type transparent ceramic at the wavelength to be used. At this time, it is preferable to thoroughly wipe and clean the optical surfaces before applying the anti-reflection coating treatment to prevent any dirt from remaining on the optical end facets, and to inspect the cleanliness using a stereoscope or microscope. Furthermore, to avoid scratching the optical surfaces or rubbing dirt on them during the wiping and cleaning process, it is preferable to select handling jigs made of soft materials and wipers that generate little dust.

[0073] The above manufacturing method is used to produce a sintered body of garnet-type composite oxide containing terbium and aluminum, and the average loss coefficient within the optical effective diameter at a wavelength of 1,064 nm for a length (optical path length) of 15 mm or more and 25 mm or less is 0.0017 cm -1 It is possible to provide a paramagnetic garnet-type transparent ceramic having a maximum insertion loss of 0.02 dB or less, a difference between the maximum and average insertion loss within the optical effective diameter of 0.02 dB or less, an extinction ratio of 40 dB or more over the entire optical effective diameter, and good optical quality even when a high-power laser with an output of 100 W or more at a wavelength of 1,064 nm is incident thereon.

[0074] The term "optically effective diameter" refers to the optically effective region (optically effective area) of the optical surface of a transparent ceramic. More specifically, in the case of a cylindrical paramagnetic garnet-type transparent ceramic, it refers to the region of the optical surface (circular surface) on the optically utilized axis, excluding the optically unusable outer edge of the end face. Here, it refers to the region excluding the outer edge of the optical surface, which corresponds to 10% of the area ratio of the optical surface, in other words, the region with an area ratio of 90% inward from the outer edge of the optical surface.

[0075] The paramagnetic garnet-type transparent ceramics obtained by the present invention preferably have an average sintered grain size (average grain size) of 10 μm or more. When the average sintered grain size of the paramagnetic garnet-type transparent ceramics is as large as 10 μm, the grain boundary area within the optical effective diameter is reduced compared to oxide sintered transparent ceramics having a sintered grain size smaller than that, which is preferable because it reduces the absolute amount of grain boundary scattering. In particular, when the average sintered grain size of the paramagnetic garnet-type transparent ceramics is 10 μm or more, it is preferable to reduce the average loss factor within the optical effective diameter to 0.0017 cm. -1 It is preferable because it can be reduced to below 0.05 μm. There is no particular upper limit to the average sintered grain size (average grain size) of the paramagnetic garnet-type transparent ceramics, but when produced under the above conditions, it is usually 35 μm.

[0076] The insertion loss is expressed in dB units as the decrease in light intensity relative to the light intensity (incident light intensity) when the ceramic is not inserted, measured by a semiconductor photodetector when 10 to 20 mW laser light with a wavelength of 1,064 nm is focused to a beam diameter of 200 to 350 μm and incident perpendicularly (in the direction of the optically used axis) on the optical surface of the target paramagnetic garnet-type transparent ceramic. The insertion loss coefficient was calculated by measuring the insertion loss while moving the laser beam incident position at a constant pitch over the entire optically effective region (optical effective diameter) of the optical surface of the target paramagnetic garnet-type transparent ceramic, calculating the average value of the insertion loss at all the measurement points, and using this average value to calculate the insertion loss coefficient using the following formula: The amount of positional movement of the laser beam in this case is preferably about half the beam diameter (100 μm). Average loss coefficient ( / cm) = (average insertion loss) / 10 × (1 cm / sample length (mm)) / log 10 e

[0077] The paramagnetic garnet-type transparent ceramics obtained by the present invention have an average sintered grain size (average grain size) of 10 μm or more, and the color center absorption due to oxygen deficiency is completely eliminated by the above-mentioned oxidation annealing treatment, so that the extinction ratio within the optically effective surface at a wavelength of 1,064 nm is improved to 40 dB or more over the entire surface in the optical path length range of 15 mm to 25 mm.

[0078] Furthermore, the paramagnetic garnet-type transparent ceramics obtained by the present invention use near-net-shape pressure-sintered compacts with a specified finished diameter after pressure sintering as described above, and therefore residual bubbles are sufficiently reduced throughout the entire region of the transparent ceramic, including the center. As a result, the difference between the maximum and average insertion losses within the optically effective surface at a wavelength of 1,064 nm is suppressed to 0.02 dB or less within the optical path length range of 15 mm to 25 mm.

[0079] Furthermore, the paramagnetic garnet-type transparent ceramics obtained by the present invention have a total light transmittance of 84.4% or more without an antireflection coating. Since the paramagnetic garnet-type transparent ceramics are intended for use in high-power laser systems of 100 W or more, it is preferable to apply an antireflection coating to both optical end faces. In this case, the antireflection coating increases the total light transmittance at a wavelength of 1,064 nm to 99.8% or more.

[0080] Note that "total light transmittance" refers to the ratio (total light transmittance) of the total intensity of all light of the target wavelength collected by the integrating sphere after passing through a transparent ceramic sample, when the transmission spectrum (light intensity) of the target wavelength measured in a blank (space) state without placing a sample in the measurement light path where the integrating sphere is installed is taken as 100%. In other words, if the intensity of the light of the target wavelength measured in the blank state (incident light intensity) is I0 and the intensity of the light collected by the integrating sphere after passing through the transparent ceramic sample is I, it can be expressed as I / I0 x 100 (%).

[0081] [Magneto-optical devices] In the present invention, it is assumed that the obtained paramagnetic garnet-type transparent ceramics will be used as magneto-optical materials, and therefore it is preferable to use the paramagnetic garnet-type transparent ceramics to form a magneto-optical device by applying a magnetic field parallel to its optical axis and then setting a polarizer and an analyzer so that their optical axes are shifted by 45 degrees from each other. That is, the paramagnetic garnet-type transparent ceramics obtained in the present invention are suitable as magneto-optical materials for magneto-optical devices, and are particularly suitable for use as Faraday rotators in optical isolators with wavelengths of 0.9 to 1.1 μm.

[0082] Fig. 2 is a cross-sectional schematic diagram showing an example of an optical isolator, which is an optical device having, as an optical element, a Faraday rotator made of the paramagnetic garnet-type transparent ceramic obtained by the present invention. In Fig. 2, the optical isolator 100 includes a Faraday rotator 110 made of the paramagnetic garnet-type transparent ceramic obtained by the present invention, and a polarizer 120 and an analyzer 130, which are polarizing materials, are provided before and after the Faraday rotator 110. Furthermore, the optical isolator 100 is preferably arranged in this order: polarizer 120, Faraday rotator 110, and analyzer 130, with a magnet 140 mounted on at least one of the side surfaces of these elements. The optical isolator 100 can also be suitably used in industrial fiber laser devices, specifically to prevent reflected light from returning to the laser light source, causing oscillation instability. [Example]

[0083] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these.

[0084] [Example 1] We obtained terbium oxide powder and scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. We also obtained liquid tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. The purity of the powder raw materials was 99.99% by mass or more, and the purity of the liquid raw materials was 99.999% by mass or more. Using the above raw materials, the mixing ratio was adjusted to obtain a final composition of Tb3Al5O 12 : An oxide starting material containing Sc (1 mass %) and Si (100 ppm) was prepared. Next, the prepared oxide starting materials were dispersed and mixed in ethanol using an alumina ball mill. The processing time was 10 hours. After that, a spray drying process was carried out to produce granular raw materials with an average particle size of 20 μm. The obtained granular raw material was subjected to uniaxial press molding using uniaxial press dies with different inner diameters to produce cylindrical compacts. Here, the inner diameters of the uniaxial press dies are shown in Table 1 as the diameters of the compacts. These pressed compacts were then subjected to isostatic pressing at a pressure of 198 MPa to obtain CIP compacts, which were then degreased in a muffle furnace at 1,000°C for 2 hours to prepare various compacts with lengths L0 shown in Table 1. Next, all the degreased compacts were placed in a vacuum furnace and sintered at 1,600°C for 3 hours at a heating rate of 100°C / h. The sintered relative densities of the obtained samples were all within the range of 94.5% to 99.0%. Each of the resulting sintered compacts was placed in a carbon heater HIP furnace and subjected to HIP treatment under Ar conditions at 200 MPa, 1,700°C, and 3 hours. As a result, each of the resulting pressure-sintered compacts was slightly grayed in appearance (due to oxygen deficiency absorption), but all were transparent. The diameter (outer diameter) and length of each of the resulting transparent pressure-sintered compacts were measured using calipers (Table 1). The outer periphery was then ground using a centerless grinder to a diameter of 5 mm (tolerance +0 / -0.05 mm). The ground and pressure-sintered compacts thus obtained were subjected to oxidation annealing treatment at 1,500°C for 6 hours in an oxygen atmosphere furnace while managing each lot, to recover from oxygen deficiency. All samples were colorless and transparent. Furthermore, each of the obtained ceramic sintered bodies was ground and polished to a length of 15 mm, and then both optical end faces of each sample were subjected to final optical polishing with an optical surface precision of λ / 8 (when the measurement wavelength λ=633 nm) to obtain a paramagnetic garnet-type transparent ceramic sample. The results of this processing are shown in Table 1.

[0085] [Table 1]

[0086] For each sample obtained as described above, the average sintered grain size (average grain size), the average insertion loss within the optical effective diameter, the insertion loss variation and average loss coefficient, and the minimum in-plane extinction ratio were measured as follows.

[0087] (Method for measuring average sintered particle size (average particle size)) The average sintered grain size (average grain size) was calculated by using a Zeiss optical microscope with a 50x objective lens to focus on the grain boundaries on the surface of each optically polished sample, printing each of the resulting images, drawing a straight line from the upper left corner of each printed image to the lower right corner, counting the length of all surface grains that intersected with the line, and finally dividing the total value by the number of grains that intersected with the line.

[0088] (Method of measuring average insertion loss, insertion loss variation and average loss factor) The insertion loss was measured using an in-house optical system that included a light source manufactured by NKT Photonics, a collimator lens, a work stage, a power meter manufactured by Gentec, and a Ge photodetector. The insertion loss was measured based on the light intensity when light with a wavelength of 1,064 nm was narrowed down to a beam diameter of 200 μm and passed through the optical system, and was calculated based on the following formula. Insertion loss (dB / sample length: 15 mm) = -10 × log 10 (I / I0) (In the formula, I represents the transmitted light intensity (the intensity of light transmitted in a straight line through a sample with a length of 25 mm), and I0 represents the incident light intensity.) Then, a mechanism was added to the work stage on which the sample was placed that allowed it to move up and down and left and right using an auto-stepping motor, and the insertion loss measurement described above was repeated while moving the sample from one end to the other in 100 μm increments, thereby measuring the insertion loss distribution over the entire optical effective diameter surface. At this time, the average value of the insertion loss data obtained at all measurement points was calculated, and the difference between this value and the maximum value was read as the insertion loss fluctuation. Furthermore, based on the average value of the insertion loss data, the average loss factor was also calculated using the following formula: Average loss coefficient ( / cm) = (average insertion loss) / 10 × (1 cm / sample length; 15 mm) / log 10 e

[0089] (Method for measuring the minimum extinction ratio within the plane) The minimum in-plane extinction ratio was measured using the system used in the insertion loss measurement described above, with a polarizer and analyzer unit added. Specifically, an in-house optical system was used, with a light source manufactured by NKT Photonics, a collimator lens, a polarizer, a work stage, an analyzer, a power meter manufactured by Gentec, and a Ge photodetector aligned in that order on the optical axis. Light with a wavelength of 1,064 nm was transmitted through the sample with a beam diameter of 200 μm. The light intensity I0' (maximum laser light intensity) was measured when the analyzer's polarization plane was aligned with that of the polarizer. The analyzer's polarization plane was then rotated 90 degrees to orthogonalize it to the polarizer's, and the received light intensity I' (minimum laser light intensity) was measured again. The minimum in-plane extinction ratio was then calculated using the following formula: Then, a mechanism was added to the work stage on which the sample was placed, which could move it up and down and left and right using an auto-stepping motor, and the extinction ratio measurement was repeated while moving the sintered sample from one end to the other at 100 μm intervals, thereby measuring the extinction ratio over the entire optical effective diameter surface, and the smallest value among these was taken as the minimum extinction ratio value within the surface. Extinction ratio (dB / sample length; 15mm) = -10×log 10 (I' / I0')

[0090] The above results are summarized in Table 2.

[0091] [Table 2]

[0092] From the above results, in the examples in which the difference between the pressure sintered finished diameter D1 and the final diameter D2 (φ5 mm) of the pressure sintered body was kept to 1.5 mm or less, the average loss factor was 0.0017 cm -1 It was confirmed that the insertion loss variation (the difference between the maximum and average insertion loss within the optical effective diameter) was 0.020 dB or less, and the minimum extinction ratio within the surface (the minimum extinction ratio within the optical effective diameter) was 40 dB or more. On the other hand, in the comparative examples in which the difference between the pressure sintered finished diameter D1 and the final diameter D2 (φ5 mm) of the pressure sintered body was 1.75 mm or more, the average loss coefficient was 0.0021 cm -1 As described above, it was confirmed that the insertion loss variation (the difference between the maximum and average insertion loss within the optical effective diameter) was 0.030 dB or more, and the minimum in-plane extinction ratio (the minimum extinction ratio within the optical effective diameter) was below 40 dB. Note that in Comparative Example 1-1, although the average grain size exceeded 10 μm, the average loss coefficient, insertion loss variation, and extinction ratio did not meet the specifications, indicating that it is difficult to obtain the desired paramagnetic garnet-type transparent ceramics when the difference between the finished diameter D1 of the pressure-sintered compact after pressure sintering and the final diameter D2 (φ5 mm) is 1.75 mm or more.

[0093] [Example 2] We obtained terbium oxide powder, yttrium oxide powder, and scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. We also obtained liquid tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. The purity of the powder raw materials was 99.99% by mass or more, and the purity of the liquid raw materials was 99.999% by mass or more. Using the above raw materials, the mixing ratio is adjusted to obtain a final composition of (Tb 0.8 Y 0.2 )3AlO 12 : An oxide starting material containing Sc (1 mass %) and Si (100 ppm) was prepared. Next, the prepared oxide starting materials were dispersed and mixed in ethanol using an alumina ball mill. The processing time was 10 hours. After that, a spray drying process was carried out to produce granular raw materials with an average particle size of 20 μm. The obtained granular raw material was subjected to uniaxial press molding using uniaxial press dies with different inner diameters to produce cylindrical compacts. Here, the inner diameters of the uniaxial press dies are shown in Table 3 as the diameters of the compacts. These pressed compacts were then subjected to isostatic pressing at a pressure of 198 MPa to obtain CIP compacts, which were then degreased in a muffle furnace at 1,000°C for 2 hours to prepare various compacts with lengths L0 shown in Table 3. Next, all the degreased compacts were placed in a vacuum furnace and sintered at 1,600°C for 4 hours at a heating rate of 100°C / h. The sintered relative densities of the obtained samples were all within the range of 94.5% to 99.0%. Each of the resulting sintered compacts was placed in a carbon heater HIP furnace and subjected to HIP treatment under Ar conditions at 200 MPa, 1,700°C, and 3 hours. As a result, each of the resulting pressure-sintered compacts was slightly grayed in appearance (due to oxygen deficiency absorption), but all were transparent. The diameter (outer diameter) and length of each of the resulting transparent pressure-sintered compacts were measured using calipers (Table 3). The outer periphery was then ground using a centerless grinder to a diameter of 5 mm (tolerance +0 / -0.05 mm). The ground and pressure-sintered compacts thus obtained were subjected to oxidation annealing treatment at 1,500°C for 6 hours in an oxygen atmosphere furnace while managing each lot, to recover from oxygen deficiency. All samples were colorless and transparent. Furthermore, each of the obtained ceramic sintered bodies was ground and polished to a length of 20 mm, and then both optical end faces of each sample were subjected to final optical polishing with an optical surface precision of λ / 8 (when the measurement wavelength λ=633 nm) to obtain a paramagnetic garnet-type transparent ceramic sample. The results of this processing are shown in Table 3.

[0094] [Table 3]

[0095] For each sample obtained as described above, the average sintered grain size (average grain size), the average insertion loss within the optical effective diameter, the insertion loss variation and average loss coefficient, and the minimum in-plane extinction ratio were measured in the same manner as in Example 1. Note that the sample length in the calculation formula was changed from 15 mm to 20 mm. The obtained results are summarized in Table 4.

[0096] [Table 4]

[0097] From the above results, in the examples in which the difference between the pressure sintered finished diameter D1 and the final diameter D2 (φ5 mm) of the pressure sintered body was kept to 1.5 mm or less, the average loss factor was 0.0016 cm -1 It was confirmed that the insertion loss variation (the difference between the maximum and average insertion loss within the optical effective diameter) was 0.015 dB or less, and the minimum extinction ratio within the surface (the minimum extinction ratio within the optical effective diameter) was 40 dB or more. On the other hand, in the comparative examples in which the difference between the pressure sintered finished diameter D1 and the final diameter D2 (φ5 mm) of the pressure sintered body was 1.75 mm or more, the average loss coefficient was 0.0018 cm -1 From the above, it was confirmed that the insertion loss variation (the difference between the maximum and average insertion loss within the optical effective diameter) was 0.025 dB or more, and the minimum extinction ratio within the surface (the minimum value of the extinction ratio within the optical effective diameter) was below 40 dB.

[0098] [Example 3] We obtained terbium oxide powder, yttrium oxide powder, and scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. We also obtained liquid tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. The purity of the powder raw materials was 99.99% by mass or more, and the purity of the liquid raw materials was 99.999% by mass or more. Using the above raw materials, the mixing ratio is adjusted to obtain a final composition of (Tb 0.65 Y 0.35 )3AlO 12 : An oxide starting material containing Sc (1 mass %) and Si (100 ppm) was prepared. Next, the prepared oxide starting materials were dispersed and mixed in ethanol using an alumina ball mill. The processing time was 10 hours. After that, a spray drying process was carried out to produce granular raw materials with an average particle size of 20 μm. The obtained granular raw material was subjected to uniaxial press molding using uniaxial press dies with different inner diameters to produce cylindrical compacts. Here, the inner diameters of the uniaxial press dies are shown in Table 5 as the diameters of the compacts. These pressed compacts were then subjected to isostatic pressing at a pressure of 198 MPa to obtain CIP compacts, which were then degreased in a muffle furnace at 1,000°C for 2 hours to prepare various compacts with lengths L0 shown in Table 5. Next, all the degreased compacts were placed in a vacuum furnace and sintered at 1,600°C for 5 hours at a heating rate of 100°C / h. The sintered relative densities of the obtained samples were all within the range of 94.5% to 99.0%. Each of the resulting sintered compacts was placed in a carbon heater HIP furnace and subjected to HIP treatment under Ar conditions at 200 MPa, 1,700°C, and 3 hours. As a result, each of the resulting pressure-sintered compacts was slightly grayed in appearance (due to oxygen deficiency absorption), but all were transparent. The diameter (outer diameter) and length of each of the resulting transparent pressure-sintered compacts were measured using calipers (Table 5). The outer periphery was then ground using a centerless grinder to a diameter of 5 mm (tolerance +0 / -0.05 mm). The ground and pressure-sintered compacts thus obtained were subjected to oxidation annealing treatment at 1,500°C for 6 hours in an oxygen atmosphere furnace while managing each lot, to recover from oxygen deficiency. All samples were colorless and transparent. Furthermore, each of the obtained ceramic sintered bodies was ground and polished to a length of 25 mm, and then both optical end faces of each sample were subjected to final optical polishing with an optical surface precision of λ / 8 (when the measurement wavelength λ=633 nm) to obtain a paramagnetic garnet-type transparent ceramic sample. The results of this processing are shown in Table 5.

[0099] [Table 5]

[0100] For each sample obtained as described above, the average sintered grain size (average grain size), the average insertion loss within the optical effective diameter, the insertion loss variation and average loss coefficient, and the minimum in-plane extinction ratio were measured in the same manner as in Example 1. Note that the sample length in the calculation formula was changed from 15 mm to 25 mm. The obtained results are summarized in Table 6.

[0101] [Table 6]

[0102] From the above results, in the examples in which the difference between the pressure sintered finished diameter D1 and the final diameter D2 (φ5 mm) of the pressure sintered body was kept to 1.5 mm or less, the average loss factor was 0.0016 cm -1 It was confirmed that the insertion loss variation (the difference between the maximum and average insertion loss within the optical effective diameter) was 0.010 dB or less, and the minimum extinction ratio within the surface (the minimum extinction ratio within the optical effective diameter) was 40 dB or more. On the other hand, in Comparative Examples 3-2 and 3-3, in which the difference between the pressure sintered finished diameter D1 and the final diameter D2 (φ5 mm) of the pressure sintered body was 2.5 mm or more, the average loss coefficient was 0.0025 cm -1As described above, it was confirmed that the insertion loss variation (the difference between the maximum and average insertion loss within the optical effective diameter) was 0.035 dB or more, and the minimum extinction ratio within the plane (the minimum extinction ratio within the optical effective diameter) was below 40 dB. Note that for Comparative Example 3-1, in which the difference between the pressure-sintered finished diameter D1 and the final diameter D2 (φ5 mm) of the pressure-sintered compact was 1.75 mm, the minimum extinction ratio within the plane exceeded 40 dB, but the average grain size was below 10 μm, and the average loss coefficient was 0.0019 cm -1 Given the above and the insertion loss variation of 0.025 dB, it was determined that it would be difficult to satisfy the desired optical specifications.

[0103] Although the present invention has been described above using the embodiments, the present invention is not limited to these embodiments, and can be modified within the scope of what a person skilled in the art can conceive, such as other embodiments, additions, changes, deletions, etc., and any aspect is within the scope of the present invention as long as it achieves the effects of the present invention. [Explanation of symbols]

[0104] 10, 90 Pressure sintered body 11, 91 Paramagnetic garnet-type transparent ceramics 100 Optical isolator 110 Faraday rotator 120 Polarizer 130 analyzer 140 Magnet D1, D1' Finished diameter after pressure sintering D2 Final diameter t, t' Cutting allowance

Claims

1. A sintered body of a garnet-type (A3B5O12) composite oxide containing at least terbium and aluminum (wherein the A site of the composition formula contains 60 mol% to 100 mol% of terbium and the B site contains 80 mol% to 100 mol% of aluminum), wherein SiO 2 1. A method for producing a paramagnetic garnet-type transparent ceramic, comprising pressure-sintering a cylindrical sintered body containing more than 0 mass % and not more than 0.1 mass % of a compound, and further subjecting the pressure-sintered body to oxidation annealing, a pressure-sintered body having a near-net-shape diameter after pressure sintering that is +0.1 mm to +1.5 mm relative to the final diameter of the paramagnetic garnet-type transparent ceramics to be obtained, the pressure-sintered body being ground to the final diameter, and then the oxidation annealing treatment is carried out.

2. 2. The method for producing paramagnetic garnet-type transparent ceramics according to claim 1, wherein said garnet-type composite oxide further contains yttrium.

3. 3. The method for producing a paramagnetic garnet-type transparent ceramic according to claim 1, wherein the average particle size of the resulting paramagnetic garnet-type transparent ceramic is 10 [mu]m or more.

4. 4. The method for producing a paramagnetic garnet-type transparent ceramic according to claim 1, wherein the length of the near-net-shape pressure-sintered compact in the optical axis direction after pressure sintering is +0.5 mm or more relative to the final length of the paramagnetic garnet-type transparent ceramic in the optical axis direction.

5. 5. The method for producing a paramagnetic garnet-type transparent ceramic according to claim 1, wherein the final diameter of the paramagnetic garnet-type transparent ceramic obtained is 3.6 to 8 mm.

6. The composite oxide is a garnet-type (A3B5O12) composite oxide containing at least terbium and aluminum (wherein the A site of the composition formula contains 60 mol% to 100 mol% of terbium and the B site contains 80 mol% to 100 mol% of aluminum), and contains SiO2 as a sintering aid. 2 a cylindrical sintered body containing more than 0 mass % and not more than 0.1 mass % of a paramagnetic garnet-type transparent ceramic, the pressure sintered body being a near-net-shape type whose finished diameter after pressure sintering is +0.1 mm to 1.5 mm relative to the final diameter of the paramagnetic garnet-type transparent ceramic to be finally obtained, the outer periphery of which is ground to the final diameter, and then an oxidation annealing treatment is performed.

7. 7. A method for suppressing the fluctuation of insertion loss within the optically effective diameter of a paramagnetic garnet-type transparent ceramic according to claim 6, wherein said garnet-type composite oxide further contains yttrium.

8. 8. A method for suppressing the fluctuation of insertion loss within the optical effective diameter of a paramagnetic garnet-type transparent ceramic according to claim 6 or 7, wherein the length of the pressure-sintered compact in the optical axis direction is +0.5 mm or more relative to the final length in the optical axis direction of the paramagnetic garnet-type transparent ceramic obtained.

9. 9. The method for suppressing the fluctuation of insertion loss within the optically effective diameter of a paramagnetic garnet-type transparent ceramic according to claim 6, wherein the final diameter of the paramagnetic garnet-type transparent ceramic obtained is 3.6 to 8 mm.

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